In disk based magnetic recording systems, data are typically stored in circular tracks. The recorded information of the disk surface is divided into sectors. A servo signal is provided on the disk to direct the actuator motor to find and then follow a track. Disk drives commonly use embedded servo information, wherein a small part of the disk surface is allocated to servo fields. The servo fields are encoded to indicate to the servo control system the position of the read/write head on the disk. The servo fields store information about the sector number, the track number, and how to center the head on a track. The track number is encoded using grey codes. The servo information is recorded onto the disk after the Head Disk Assembly (HDA) has been assembled.
Information is stored on the disk in the form of magnetic transitions. Each bit of information stored on the disk corresponds to one magnetic transition. A read head generates electrical signals corresponding to the magnetic transitions. A "1" may be used to designate the presence of a magnetic transition, and a "0" to designate the lack of a magnetic transition. The read head generates either a positive or negative pulse for each magnetic transition depending on the polarity of the transition. Data are read from the disk by processing transition responses.
A peak detector circuit attached to the read head analyzes the analog signal generated by the read head to identify the presence of pulses which indicate magnetic transitions. One conventional analog peak detection method is to differentiate the signal and detect zero crossings of the signal derivative. The signal derivative is zero for local minimums and local maximums. The amplitude of the signal where the derivative is zero is then compared to a threshold level to identify peak samples. Such peak detection methods typically have a sample comparison window two to three samples wide.
A conventional digital peak detection method is to convert the analog samples to digital samples, and then compare a sample to the previous sample and subsequent sample. If the sample is greater than the previous and subsequent sample then the sample is compared with a threshold level. If the sample exceeds the threshold then the sample is identified as a peak. Similar to the conventional analog peak detection system such digital methods typically have a peak comparison window that is three samples wide. One problem with such detection methods is that noise spikes can cause two or more peaks to be reported within a timing window in which there is only one recorded peak. Such false peaks reduce operating efficiency where they are detected and trigger a second reading of the data. When such errors go undetected they can cause system failures.
Peak detectors typically use a variable gain amplifier ("VGA") to amplify the read signal so that the pulses approximate a desired amplitude to optimize pulse detection. Conventional analog peak detectors use the charge on a capacitor to control the VGA gain. One capacitor is used for reading servo signals in servo mode and one capacitor is used for reading data signals in data mode. The capacitors are coupled to the VGA control terminal using a multiplexer to switch between the two capacitors. An automatic gain control (AGC) feedback loop is used to control the VGA gain. When a pulse is detected above the AGC target amplitude, a small amount of charge is discharged from the gain control capacitor to reduce the VGA gain. A resistor coupled to the gain control capacitor is used to charge the capacitor. When a series of pulses are below a minimum qualified pulse amplitude the resistor charges the capacitor which increases the VGA gain and thereby increases the pulse amplitude. One problem with these conventional analog peak detectors is that if the read head goes through a damaged data field where none of the pulses exceed the qualified pulse amplitude level then at the end of the data field the gain is generally increased to a higher level then is desired for the subsequent intact data field. This causes delays while the AGC loop recovers.
At the end of a servo field the servo gain control capacitor stores the gain value for the next servo field. Similarly the data gain control capacitor stores the gain for the next data field. During these storage periods the value of the stored gain deteriorates due to leakage of the capacitor.
Thus there is a need for an improved peak detection system that provides enhanced performance and overcomes these and other problems of the prior art.